1. Field of the Invention
The present invention relates to relay circuits, and, in particular, to fast relay circuits with reduced bounce and low power consumption.
2. Description of the Related Art
It is well known to use relay circuits to close output contacts that are electrically isolated from the relay circuit. Referring now to FIG. 1, there is shown a prior art relay circuit 100. As is known to those skilled in the art, a relay circuit contains a relay 101, which comprises inductor coil L having internal resistance R.sub.L. When a voltage V.sub.1 is applied to relay 101, current I.sub.1 passes through inductor L, inducing a magnetic field which forces output contact 120 to close. In this manner, as is well known to those skilled in the art, a voltage V.sub.1 applied to relay 101 can close an electrically isolated circuit containing output contact 120.
Relays are often used as protective relays to protect power systems and thus require fast operating times. To force output contact 120 to close more quickly in response to input voltage V.sub.1, V.sub.1 may be increased and a resistor R.sub.A added in series with relay 101, as shown in FIG. 1. Resistor R.sub.A reduces the amount of input current I.sub.1 drawn by relay 101, but the speed of relay 101 is increased because the time constant L/R=L/(R.sub.L +R.sub.A) is decreased. If current I.sub.1 is driven by a larger voltage V.sub.1, inductor L is energized more quickly so that output contact 120 closes more quickly. If the power delivered by voltage source V.sub.1 is doubled, for example, the time required to close output contact 102 is reduced. However, much of the increased power is wasted in resistor R.sub.A.
When output contact 120 closes more quickly because the input power is increased, output contact 120 has a greater tendency to bounce since it slams shut with greater force and speed. Thus, in the prior art, relay circuits were speeded up by increasing the power delivered to the circuit, which also increased the tendency of the output contact to bounce. Increased bounce and increased power requirements are undesirable characteristics for many applications.
It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the known art and to provide a relay circuit that more quickly closes an output contact.
It is a further object of this invention to provide such a fast relay circuit that has low power consumption and that also reduces output contact bounce.
Further objects and advantages of this invention will become apparent from the detailed description of a preferred embodiment which follows.